Large-scale Monsoon Circulation Research Articles

1997年インドモンスーンに対するエルニーニョの影響

The 1997 Indian Summer Monsoon is investigated in the context of interannual variations. It is evidenced that the 1997/98 El Nino, one of the strongest in history, suppressed the large-scale monsoon circulations. On the contrary, however, the Indian summer monsoon rainfall was not much reduced. In fact, it slightly exceeded the climatological seasonal average. An inquiry into circulation and precipitation data indicates that it was the intraseasonal variability of the monsoon system that brought the above-normal rainfall over India. Furthermore, it is shown that the 1997 El Nino not only suppressed the large-scale Asian monsoon circulations, but also produced a convectively unstable area off the east coast of Somalia through the modifications in sea surface and lower tropospheric conditions. Anomalous convection was triggered, amplified over this area, then intruded northward to the Indian subcontinent as it propagated eastward. A moisture budget analysis using 41 years (1958-98) of the NCEP/NCAR reanalysis data set confirmed that the transient part of moisture transport anomaly in the 1997 summer was larger than the stationary one, contributing positively to the small positive rainfall anomalies over India. The 1997 summer appears to be one of the singular cases in the context of the general positive correlation between the monsoon circulation and precipitation indices over the recent 41 years.

Intraseasonal oscillations and interannual variability of surface winds over the Indian monsoon region

The role of intraseasonal oscillations (ISOs) in modulating synoptic and interannual variations of surface winds over the Indian monsoon region is studied using daily averaged National Centers for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) reanalyses for the period 1987–1996. Two dominant ISOs are found in all years, with a period between 30–60 days and 10–20 days respectively. Although the ISOs themselves explain only about 10–25% of the daily variance, the spatial structure of variance of the ISOs is found to be nearly identical to that of high frequency activity (synoptic disturbances), indicating a significant control by the ISOs in determining the synoptic variations. Zonal and meridional propagation characteristics of the two modes and their interannual variability are studied in detail. The synoptic structure of the 30–60 day mode is similar in all years and is shown to be intimately related to the strong (‘active’) or weak (‘break’) phases of the Indian summer monsoon circulation. The peak (trough) phase of the mode in the north Bay of Bengal corresponds to the ‘active’ (‘break’) phase of monsoon strengthening (weakening) the entire large scale monsoon circulation. The ISOs modulate synoptic activity through the intensification or weakening of the large scale monsoon flow (monsoon trough). The peak wind anomalies associated with these ISOs could be as large as 30% of the seasonal mean winds in many regions. The vorticity pattern associated with the 30–60 day mode has a bi-modal meridional structure similar to the one associated with the seasonal mean winds but with a smaller meridional scale. The spatial structure of the 30–60 day mode is consistent with fluctuations of the tropical convergence zone (TCZ) between one continental and an equatorial Indian Ocean position. The 10–20 day mode has maximum amplitude in the north Bay of Bengal, where it is comparable to that of the 30–60 day mode. Elsewhere in the Indian Ocean, this mode is almost always weaker than the 30–60 day mode. In the Bay of Bengal region, the wind curl anomalies associated with the peak phases of the ISOs could be as large as 50% of the seasonal mean wind curl. Hence, ISOs in this region could drive significant ISOs in the ocean and might influence the seasonal mean currents in the Bay. On the interannual time scale, the NCEP/NCAR reanalysed wind stress is compared with the Florida State University monthly mean stress. The seasonal mean stress as well as interannual standard deviation of monthly stress from the two analyses agree well, indicating absence of any serious systematic bias in the NCEP/NCAR reanalysed winds. It is also found that the composite structure of the 30–60 day mode is strikingly similar to the dominant mode of interannual variability of the seasonal mean winds indicating a strong link between the ISOs and the seasonal mean. The ISO influences the seasonal mean and its interannual variability either through increased/decreased residence time of the TCZ in the continental position or through occurrence of stronger/weaker active/break spells. Thus, the ISOs seem to modulate all variability in this region from synoptic to interannual scales.

気象庁全球モデルでシミュレートされた北半球夏の熱帯の30-60日季節内振動

The JMA global model at T106 horizontal resolution has been integrated for 10 years with observed SST. Intraseasonal variations in the tropics during N.H. summer have been examined using 30-60 day filtered anomalies from the model simulated results. Eastward propagation of velocity potential anomalies on a 30-60 day time scale has been noticed throughout the equatorial region with a period of about 38 days. Composite analyses have been made using the principal components of an EOF analysis of the upper tropospheric velocity potential anomalies. The observed characteristics of the oscillation have been simulated by the model reasonably well. The equatorial oscillation modulates the strength of the Indian monsoon and plays an important role in deftning its intraseasonal variability on a 30-60 day time scale. The model simulates well the northward propagation of the rainfall anomalies from the equatorial Indian Ocean to the Indian region. An EOF analysis has been made using the filtered rainfall anomalies in the Indian longitudes. Composite analyses of rainfall, wind and surface fields have been made using the principal components of the EOF analysis. Circulation features in the different phases of the northward propagation resemble that of active and weak monsoon conditions and agree well with the observed analyses. It it noticed that latent heat flux variations over the sea on a 30-60 day time scale associated with the modulation of large scale monsoon circulation by the eastward propagating equatorial intraseasonal mode play a more important role than the land surface processes in the northward propagation.

Interaction of secondary circulations with the summer monsoon and diurnal rainfall over Hong Kong

A complete yearly record (1988) of surface measurements is used to examine the atmospheric diurnal secondary circulations over the entire area of Hong Kong in conjunction with spatial and temporal variations of surface temperature, wind speed and rainfall. Evidence of atmospheric diurnal secondary circulations is found at 10 sites. The occurrence of a summer morning rainfall maximum over the coast results from the interaction of the large-scale summer monsoon and local mesoscale secondary circulations. The afternoon onshore secondary circulation accelerates the advection of warm, humid unstable air and, coupled with the upward orographic lifting, produces enhanced rainfall along windward mountain ridges.

Effect of counter-gradient in the computation of turbulent fluxes of heat and moisture in a regional model

The counter-gradient terms in the computations of turbulent fluxes of heat and moisture have been included in the PBL parameterization of a regional model for monsoon prediction. Results show that inclusion of counter-gradient terms has a marginal impact in the prediction of large scale monsoon circulation and rainfall rates.

Satellite observations of the southern hemisphere monsoon during Winter MONEX

Time-longitude strips of infrared GMS satellite imagery have been used to study the southern hemisphere summer monsoon during Winter MONEX. This representation reveals a sudden monsoon onset in late December characterized by a transition to spatially organized convection. Active and break regions of the monsoon show a strong coherence in time but are very longitudinally dependent. The latitudinal dependence, on the other hand, is quite weak. The dominant spatial scale over which changes in convection take place is of order 35 degrees longitude by 15 degrees latitude. These synoptic scale convective regions can move in either the eastward or westward direction. Typical phase speeds of movement range between 3 and 8 degrees longitude per day. Tropical cyclones in the region are revealed to be embedded in the large-scale monsoon circulation. DOI: 10.1111/j.1600-0870.1983.tb00196.x